[dart] Do not deflake passing tests

Passing tests do not block the CQ, even if they are flakes. The extra
deflaking reduces noise in the aggregated test results (e.g. a test
might show up as Timeout->Pass even though it is a flaky pass). New
tests, which are by nature changes, are only deflaked if failing. This
can slip through some flaky passing tests but they would not have
blocked the CQ from landing even if detected as flakes.

This can save hours of cycle times in cases where bots are in short
supply and deflaking shards have high pending times or when deflaking
frequently timing out tests (they take 5 * timeout to run through
deflaking).

Change-Id: I50dd032af6d8d1b1d356d23d9f5d0ba7c8b5e8a9
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/166080
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
17 files changed